(a) Field of the Invention
The present invention relates to an analog-to-digital converter (A/D converter) having a dynamic encoder and, more particularly to an A/D converter having a dynamic encoder for encoding a thermometric code into a binary code.
(b) Description of the Related Art
A/D converter is widely used to converting an input analog signal to a digital code which can be processed by a computer system. Referring to FIG. 1 showing a conventional A/D converter called a flash A/D converter in JP-A-8-36466 (from Texas Instrument), wherein an input analog signal of a thermometric code is directly converted by using a dichotomizing search technique to output a binary code without generating an intermediate code. The flash A/D converter includes a comparator array 51 including a plurality of comparators 52, and dichotomizing search encoder 53 including a plurality of D-flip-flops 54 and 56, and a plurality of multiplexers 55. The A/D converter has an advantage of reduction of the maximum sparkle error to a minimum. That is, the A/D converter has an advantage in that an input thermometric code having a bubbling error provides an output signal with an error as low as within one bit after error correction compared to the case in which an ideal thermometric code is input.
Specifically, referring to FIG. 2 showing the results of conversion after error correction in the A/D converter of FIG. 1, an input analog signal of "level four" or a digital code "100" corresponding to an ideal thermometric code of (1111000) is input to the A/D converter, which achieves a binary code (011) after correction of error caused by bubbling. That is, the conventional A/D converter achieves an error within 1-bit level after the error correction.
The conventional flash A/D converter using a dichotomizing search technique as described above, however, has a problem in that the conversion rate or conversion speed thereof reduces in proportion to the increase of the number of bits of the input thermometric code. This is because the dichotomizing search technique in its nature can start the search for m-th bit in an N-bit code (0.ltoreq.m.ltoreq.N-2) only after the search for (m+1)-th bit and more significant bits is completed.
In other words, N-bit code of the output digital signal requires N times for search by the A/D converter because of the inter-bit dependency in the dichotomizing search technique, which limits the conversion rate of the A/D converter.
The conventional A/D converter has another problem in that the circuit scale and the power dissipation thereof increases with the increase of the number (N) of bits to be converted. This is partly because there is a phase difference between the most significant bit (MSB) and the least significant bit (LSB) of the code after the conversion, which requires timing flip-flops or latches in number corresponding to the number (N) of bits, partly because the search results for m+1)-th bit and more significant bits must be stored during the search for m-th bit, which requires the latches for storage in number corresponding to number of (2.sup.N -1), and partly because the dichotomizing search requires switching circuits in number corresponding to ##EQU1##
That is, if the number of bits is N, the A/D converter includes elements in number equal to: ##EQU2##
provided that D-flip-flops and multiplexers are implemented by eight elements and four elements, respectively. If N=3, for example, then the number of elements is as large as 96.